MPI_Probe sample

test.cpp

#include <mpi.h>
#include <cstdio>
#include <vector>

int main(int argv, char **argc){
	MPI_Init(&argv, &argc);
	int rank;
	MPI_Comm_rank(MPI_COMM_WORLD, &rank);
	int send_buf[] = {3,5,3,1,6,7,8,8};
	int count = sizeof(send_buf)/sizeof(int);

	if (rank==0){
		printf("I will send %d data\n",count);
		printf("Here are the data\n");
		for(int i=0;i<count;i++){
			printf("%d:%d\n",i, send_buf[i]);
		}
	  MPI_Send(send_buf, count, MPI_INT, 1, 0, MPI_COMM_WORLD);	
	}

	MPI_Barrier(MPI_COMM_WORLD);

	if (rank==1){
		MPI_Status st;
		MPI_Probe(0, 0, MPI_COMM_WORLD, &st);
		int size = st._ucount/sizeof(int);
		printf("I will receive %d data from rank %d\n",size, st.MPI_SOURCE);
	  std::vector<int> recv_buf(size);
	  MPI_Recv(recv_buf.data(), size, MPI_INT, 0, 0, MPI_COMM_WORLD, &st);	
		printf("Here are the data.\n");
		for(int i=0;i<size;i++){
			printf("%d:%d\n",i, recv_buf[i]);
		}
	}
	MPI_Finalize();
}

$ mpic++ test.cpp
$ mpirun -np 2 ./a.out
I will send 8 data
Here are the data
0:3
1:5
2:3
3:1
4:6
5:7
6:8
7:8
I will receive 8 data from rank 0
Here are the data.
0:3
1:5
2:3
3:1
4:6
5:7
6:8
7:8